I once suggested Plastigauge on another forum and was told it does not work. It's not made for this application. It is made for crank pins and rod bearings and it works there. This may not make a lot of sense but people who supposedly know say it won't work.

Thing is, the solder diametre needs to be close to the ideal shim thickness so as to not bow the tranny cover when the screws are torqued down.

yeah, that was my thought too. I was thinking of trying to make a plait or twist of thinner soldering wire first, and try to use that to determine where your ballpark is, then see if you can get a thicker strand of solder that is just too thick. I was imagining it taking a few tries to raise (or destroy) the confidence level.
For putty, you could try that 'knead it' epoxy stuff (often used for metal repair). It is about the consistency you want- only problem is that the couple of tubes I've owned have all set-up pretty quickly. There must be a slow cure version?

I once suggested Plastigauge on another forum and was told it does not work. It's not made for this application. It is made for crank pins and rod bearings and it works there. This may not make a lot of sense but people who supposedly know say it won't work.

As far as I know, the way to use plastigauge is to measure the width of the squashed plastigauge after it has been removed from the test location (big ends etc) The specific size of plastigauge used can be cross referenced with the 'squashed width' to determine clearance.

Just had a little play with the donor cover and found that I can get 2 thou difference with the depth gauge by moderately pressing on the cover.
Success with the solder depends on whether the solder can create as much resistance as my pushing on the cover...

The two options I see are starting with the solder diametre as close as possible to the correct shim size, and/or use a softer solder.

I put the cover plate back on with very close to the shim stacks noted above. All good until I got towards the last three fasteners (I called them fasteners to avoid the bolt, screw, cap screw, machine screw thingy)
The input shaft tightened up. WTF? this, after 15,000,000 measurements.

Well off came the cover again and out came approx .02mm from each stack.
Frustrating trying to come up with the numbers again from a choice of about 16 shims.

Cover went back on, all buttoned up and a nice free input shaft. Minus the input seal.

I've had a couple of rides so far and all seems well. Smooth positive shifting with no false neutrals.

The taller 5th is a gem! The RS has plenty of power to pull with the reduced rpm.
With a 33/11 I'm quite sure 130kph on the speedometer ran @ 5000.

Now, 130kph is running @4500
120 @ 4000
100@ 3500.....used to be 3800

That appears to be a difference of 4-500 rpm @ 130 and 300 rpm @ 100

Bottom line, I like it.
All my fun in the mountain twisties is 2nd 3rd and 4th so nothing lost there.

Cross my fingers that it will do another 13 years before showing chips on the magnetic plug.